Scientists believe they are what's known as 7-11 crabs, which are native to Hawaii.

But the mystery remains -- why are so many turning up now?

"It's probably something ecological because it's happening on such a large scale," Waikiki Aquarium Director Andrew Rossiter told KITV. "You say it's pollution or it may be a pulse of warm water, or a pulse of cold. If it were any of these three it would happen to other species as well, but in this case, it's just the 7-11 crab."

The crabs can grow to be quite large and can be eaten by humans. However, many won't survive because fish or birds will eat them as they wash up on the shore, Rossiter said.

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